Update golangci-lint to v2.12.2 with canonical config

Bump the golangci-lint Docker image pin in Dockerfile and the
release-archive sha256 pins in script/bootstrap from 2.11.3 to
2.12.2, and replace .golangci.yml with the canonical config. The
canonical config moves lll/funlen/cyclop/dupl settings from the
top-level linters-settings key (ignored by the v2 schema) to
linters.settings, so those thresholds now actually apply.

Fix all findings the newly applied thresholds surfaced:

- lll: wrap or shorten seven over-length lines (struct tag
  comments, test logger construction, a func signature, and a
  nosec comment)
- goconst: use http.MethodPost/http.MethodPut and new shared
  constants for repeated test strings; add tmplKeyError and
  tmplKeyWebhook constants for template data keys in handlers
- dupl: merge buildHTTPTargetConfig and buildSlackTargetConfig
  into a parameterized buildURLTargetConfig; drop the duplicate
  iWebhookDB test helper in favor of testWebhookDB; extract
  shared helpers in middleware and session tests

internal/delivery/target_http.go

@@ -495,5 +495,5 @@ func applyRequestHeaders(
 func executeHTTPRequest(
 	client *http.Client, req *http.Request,
 ) (*http.Response, error) {
-	return client.Do(req) //#nosec G704 -- URL validated by parseHTTPConfig/parseSlackConfig and SSRF-safe transport
+	return client.Do(req) //#nosec G704 -- validated URL, SSRF-safe transport
 }